The invention relates to the removal of sludge from underwater deposits.
Sludge can build up over time on the floors of artificial containers for water such as tanks or ponds or on the floors of natural bodies of water such as lakes and oceans. Removal of the sludge may be desirable to increase the volume or flow of water, to remove pollutants contained in the sludge, to collect the sludge for use as a resource or for other reasons.
The source of the sludge may be natural, e.g. from settlement of particles suspended in the water, or may be artificial, e.g. as a result of mining or drilling. It may comprise particles of a well-defined size or of a range of sizes. The size or range of sizes may vary substantially between different deposits. The sludge may contain varying proportions of water within its structure. As a result of the pattern of deposition, the sludge may be uniform or stratified. As a result of physical or chemical processes following deposition, the particles may be bonded together in a consolidated mass or they may be held together only by gravity. For the purposes of this specification, the term “sludge” should also be taken to include dense liquids and semi-solids such as wax that are not particulate in nature but have similar properties and, in particular, are susceptible to fluidization by the use of pressurized water.
One example of sludge that requires removal is an accumulation on the floors of the ponds in which nuclear waste is stored. The ponds are artificially constructed so the floors can be assumed to be flat and even but the presence of larger items dropped into the ponds cannot be ruled out. Because such sludge is radioactive, it requires efficient collection and careful disposal, and this must be achieved remotely from the presence of a human operator. Such sludge tends to have built up slowly over a long period and is therefore stratified and may be relatively strongly bonded. Previous attempts to remove sludge in this situation have achieved poor collection rates of less than 15%.
Another example of sludge that requires removal is an accumulation on the sea floor around oil or gas wells as a result of materials that have been spilled during the drilling and extraction processes. Sludge in this situation tends to be soft but poorly structured and it may form deep deposits. The natural sea floor on which the deposit rests may be uneven and its contours will likely be unknown.